A system and method for microlithographic writing of a substrate is previously known from e.g. EP 0 467 076 by the same applicant. An example of a system for microlithographic writing, as is shown in FIG. 1, comprises a light source 1, such as a laser, one or more reflecting mirrors 2 to direct the beam in the intended direction, a first lens 3 to contract the light beams, a modulator 4 to produce the desired pattern to be written, and a second lens 5 after the modulator. The modulator is controlled according to input data. A deflector is used to scan the beams on the substrate 11, and lenses 7, 8, 10 are used to contract and focus the beams on the substrate. The deflector could be a movable reflective element, such as a mirror. However, several functionally equivalent scanners such as acusto-optic deflectors etc. could also be used. Further, the substrate is preferably arranged on an object table.
A relative motion between the lens 10 and the table (stage) could be provided by arranging at least the objective lens 10, and preferably the lenses 7–8 and the deflector 6 as well, on a carrier. Hereby, the carrier could be moved relative to the substrate, and relative to the light source in a direction essentially perpendicular to the scanning direction.
If such a prior art system should be used with several beams concurrently being directed to the substrate, so called multi-beam writing, in order to improve the writing speed, several problems would arise.
It is important to obtain a gauss stationary illumination in the lens stop, but this proves difficult to obtain when using multi-beam in combination with a movable carrier. If such illumination is not obtained, beams will impinge on the substrate at different angles, whereby small differences in focus on the substrate, which is inevitable, will result in detrimental variations in the scanning lengths. For several applications, a maximum variation in scanning length of 20 nm is required. Therefore, a high degree of telecentricity, i.e. the beams being parallel during the scanning, is required. To obtain this it is important that the beams are superposed on each other in the deflector, but this proves to be difficult to obtain due to the varying distance between the deflector on the carrier and the modulator.
Specifically, using multi-beam writing in a pattern generator where part of the optical path is movable will cause problems. FIG. 2a illustrates a pattern generator where the movable optical path is in an intermediate position. In this case, the beams will coincide in the deflector 6, and a satisfying telecentricity will be obtained. However, when the carrier is moved to an end position, as is illustrated in FIG. 2b, the beams will not coincide in the deflector, but be displaced from each other. Hereby, the beam path in the objective lens 10 will be oblique, and the telecentricity will be severely deteriorated. There will also be a loss in radiation energy, since part of the beams will be displaced from the working part of the lenses.